A rotation detection device of this kind has been suggested, which includes a ring-shaped magnetic pulse generating means such as, for example, a magnetic encoder having magnetic pole pairs in a direction circumferentially thereof and for generating magnetic pulses, and a plurality of detecting elements for detecting the magnetic pulses, substantially aligned in a direction circumferentially thereof and associated with the magnetic pulse generating means and is so designed that the absolute angle can be detected by calculating the respective output signals generated from the detecting elements. (See, for example, the Patent Document 1 listed below.).
Also, as a different rotation detecting device, the rotation detecting device has been suggested (in, for example, the Patent Document 2 listed below), in which respective angles of rotation of two auxiliary gears are utilized to calculate the absolute angle of a main shaft.                [Patent Document 1] JP Published Int'l Application No. 2002-541485        [Patent Document 2] JP Patent No. 3792718        
It has, however, been found that in the case of the rotation detecting device disclosed in the Patent Document 1 listed above, it is difficult to detect the absolute angle with the use of the magnetic encoder and to secure a rotation pulse of a high resolution.
Also, in the case of the rotation detecting device disclosed in the Patent Document 2 listed above, the structure is complicated and cannot therefore be manufactured in a compact size.
The inventors of the present invention have suggested the rotation detecting device capable of detecting the absolute angle with high precision, which is of the following structure.
In this rotation detection device, the use is made of a plurality of magnetic encoders of a ring shape arranged coaxially and having different numbers of magnetic poles, a plurality of magnetic sensors each operable to detect the magnetic field of the corresponding magnetic encoder and having a function of detecting positional information within a single magnetic pole of the corresponding magnetic encoder, a phase difference detector for determining the phase difference of magnetic field signals detected respectively by the magnetic sensors, and an angle calculator for calculating an absolute rotation angle of the magnetic encoders based on the detected phase difference.
In the case of the rotation detecting device of the structure described above, if rotation is made using a magnetic encoder having 12 magnetic pole pairs, and a magnetic encoder having 13 magnetic pole pairs is 13, the phase difference corresponding to one magnetic pole pair for each complete rotation occurs between the two magnetic sensors used to detect those magnetic fields. Accordingly, by detecting such phase difference with the phase difference detector and by using the angle calculator based on such phase difference, the absolute angle during one complete rotation can be calculated. Also, since each of the magnetic sensors has a function of detecting information on a position of the respective magnetic encoder within a single magnetic pole, the absolute angle can be detected with high precision. Also, the construction can be simplified.
However, in the case of the rotation detecting device of the structure described above, if the position at which the magnetic encoders are fitted deviates, the absolute angle cannot be detected with high precision.